1. Field of the Invention
This invention relates to a varifocal lens system for high magnifications which is used as a projector lens at a finite distance in an enlarger or the like.
2. Description of the Prior Art
In an enlarger for general use for printing images on photographic paper, a fixed-focus lens is used as the projector lens, and the relative distances between the negative film, the projector lens and the photographic paper are changed. However in an enlarger for commercial use, generally a varifocal lens is used as the projector lens and the positions of the negative film and the photographic paper are fixed.
As such a varifocal lens, there has been known a lens system having a fixed-focus lens which is combined with one or more so-called diopter lens(es) as required. In the lens system, the focal length is changed by changing the combination of the diopter lenses to obtain a desired magnification.
However such a varifocal lens is disadvantageous in that the combination of the diopter lenses is complicated when the change in the focal length is large and the quality of printed image deteriorates, and that it takes a long time to adjust: the position of lenses when setting the magnification to a desired value.
There has been proposed a varifocal lens system which consists of a negative lens group and a positive lens group and in which the focal length is changed by changing the distance between the two lens groups. (See Japanese Unexamined Patent Publication No. 61(1986)-129613).
With this varifocal lens system, printed images of high quality can be obtained and at the same time, the diopter lenses can be eliminated, thereby reducing the cost.
However the varifocal lens system consisting of two lens groups is disadvantageous in that the distance between the lens system and the negative film (finite back) becomes too short, e.g., not longer than 90 mm at high magnifications. When the finite back is short, it becomes difficult for the operator to directly see the negative film due to a light measuring mirror inserted between the lens system and the negative film for color correction, which makes difficult the printing operation.